1. Field of the Invention
The present invention relates to methods of obtaining phase information in Magnetic Resonance Imaging (MRI) and a related Apparatus.
2. Discussion of Related Art
Magnetic Resonance Imaging (MRI) is a technique for imaging a material by exciting nuclei in the material when the material is in a strong magnetic field. The strong magnetic field is known as a static magnetic field. Nuclei are then excited by a radio frequency magnetic field. Positional information is obtained by the generation of magnetic field gradients. A voltage induced in a receiver coil after the gradient field is removed is measured. It is the information derived from the induced voltage which is used, with sophisticated computing systems, to produce an image which is readily recognised by the eye.
Other information exists in the induced voltage signal. Information about the phase of an induced voltage signal is useful in distinguishing two materials (a and b) whose induced voltage S.sub.a and S.sub.b are of similar magnitude.
Magnetic Resonance Imaging (MRI) is therefore a powerful medical imaging technique which generates data which contains phase as well as magnitude information, and essentially depicts the magnitude and phase of the transverse nuclear magnetisation within 20 an elemental unit volume called a voxel. However, when the image is displayed, the phase of the data is often removed, to avoid image artifacts, which are often attributable to related motion between two or more material types being imaged. In this aspect, MRI has parallels with optics, because light is also an electromagnetic wave having both an amplitude and a phase. In optics, as in MRI, the phase is often discarded in imaging methods. For instance a photographic emulsion is sensitive only to the intensity of the light.
An example of an MRI technique which uses both voltage phase and magnitude information is described in a letter in Phys. Med. Biol. 1984 Vol. 29 No. 7 at pages 891-898.